Continental drift is the theory that continents move around on the surface of the earth.
For many years, it was thought that the continents were fixed in place. But growing evidence has shown that they are slowly drifting on the earth’s surface, and were even once a single landmass.
Here is an illustration by Antonio Snider-Pellegrini from 1858 showing how the
Atlantic Ocean formed when North and South America separated from Europe and Africa.
In the 1920's, German meteorologist Alfred Wegener showed how, based on their shape, all the continents could have easily fitted together in a supercontinent he called Pangea:
Pangea, which started splitting up almost 200 million years ago.
Laurasia and Gondwanaland
Wegener also thought that Pangea broke into two smaller pieces (called Laurasia and Gondwanaland) and then further separated to give the smaller land masses we now have:
Wegener found other types of evidence that supported his hypothesis.
- Rock types and fossils that matched each other across continents as if they were once joined together. Examples were found between north Africa and the east coast of North America. Mesosaurus fossils for example, have been found on both the southern sections of South America and Africa.
- Sequences of rock layers also matched between the eastern side of North America (the Appalachians) and appear to have a similar age and structure to those that make up the British Isles and Scandinavia in western Europe.
- Even evidence showing the direction of movement of glaciers over South America, Africa, and Antarctica, indicate that could have once been covered by a continuous continental ice sheet.
Slow to Accept
Once Wegener's ideas became published and translated into English, many scientists were quick to doubt and criticize his theory. One criticism was that he could provide no accepted mechanism that caused continents to move. Wegener suggested that the tidal pull of the Moon could be a factor and that the continents could push through the crust, almost like a ship breaking through ice. The majority of the scientific community however, were not ready to accept these ideas during the 1920's.
Despite criticism, Wegener's ideas still sparked curiosity among some scientists.
Alexander du Toit, a South African geologist, explored a large portion of southern Africa and found many of the continuing patterns in rock units and layers in the geologic record. He continued mapping the continents more accurately by using the continental shelves instead of the actual coastlines, thus giving more validity to Wegener's claims.
And also Scottish geologist Arthur Holmes suggested the movement may be caused by heat (convection currents) in the mantle, the huge layer the crust actually sits on.
Nowadays, with exploration technologies such as drilling of the sea floor and accurate positioning using satellites, a wealth of data has lead to new theories and models explaining the history of continental movement.
We now know the history of continental movement. Pangea was formed about 300 million years ago and began breaking up 200 million years ago. This breakup occurred in three phases:
- About 175 million years ago (early to middle Jurassic on the Geologic Time Scale) Pangaea first began to break and form the two new supercontinents Laurasia and Gondwanaland
- 150–140 million years ago (early Cretaceous) Gondwanaland divided to form the continents of Africa, South America, India, Antarctica, and Australia
- 60–55 million years ago (early Cenozoic) Laurasia broke into North America and Greenland, both of which separated from Europe
Geologists even know the current directions in which plates are moving. And it is possible to measure how fast plates are moving (speeds range from 0 to 100 mm per year, depending on the plate).
Actually, the average plate movement is similar to the rate our fingernails grow: 1 to 2 cm per year.
Example: North America and Europe are known to be drifting apart at about 2.5 cm per year.
Not very fast, but over millions of years it adds up!
And in the future? Continental Drift will continue, and in the distant future there may be more breakups:
- Africa may split apart along the Great Rift Valley,
- And California may break away and become an island!